Dual-WAN failover problem

[Vile]

I have dual WAN configuration with primary WAN as PPPoE connection (DHCP) and secondary with static public IP address(es).
In case of failure of primary WAN, secondary have became unavailable from outside. No ping, no inbound NAT.
Outgoing NAT was working fine though.
As soon as primary connection was restored, secondary one also have became available from outside almost immediately.

Can you suggest how can I fix this behaviour?